1. Field of the Invention
The present invention relates to ultrasonic probes and, more particularly, to motorized transducer ultrasonic probes for trans-esophageal imaging.
2. Background of the Invention
Commercial transesophageal (TE) ultrasonic probes are typically provided with either a biplane or multiplane phased array transducer mounted at the distal tip of the endoscope. The biplane device is based on the integration of two phased array transducers which have the acoustic azimuth planes thereof oriented perpendicular to each other, while the multiplane device uses a single phased array transducer which is rotated around the Z-axis thereof (i.e., in the wave propagation direction). The flexibility and ease of use thereof make multiplane transducer probe the preferred instrument for use in transesophageal examinations in spite of the relatively high cost of such apparatus.
The terms scanhead, transducer tip housing, distal housing and distal tip are all sometimes used for designating the sensor compartment or section wherein the ultrasonic transducer is located. In TE applications, the dimension of the scanhead must be relatively small and, in general, must not exceed 20-25 mm in length and 12-14 mm in diameter, in order to avoid patient discomfort.
Currently, the fabrication of multiplane or 3D phased array TE probes is a highly complicated operation because of the complex components needed to provide precise rotation of transducer. Further, in order to keep the size of the scanhead of the TE device small enough to enable easy insertion thereof into the esophagus, the rotary drive means for the transducing device is typically remotely located in the handle of the endoscope in that more space is available there.
Generally speaking, the transducer rotary drive means can be an electric motor or a manually actuated device, respectively associated with a flexible drive shaft or cables. In case of motor drive device, the rotary drive shaft is provided in an elongated anti-torsion rod or tube and extends through the flexible tubular portion of the endoscope to connect to the transducer. In the handle of endoscope are also provided all of the controls for steering and positioning the scanhead. Transmission of the controls exerted from the handle to the scanhead of the endoscope is typically effected through a set of cables secured to the two respective parts. A bendable coupler or connection is provided between the scanhead and the flexible tubular portion of the endoscope to effect this operation.
There are numerous documents and literature references disclosing manufacturing and construction techniques for ultrasonic TE imaging endoscopic probes. These documents include, for example, U.S. Pat. No. 4,543,960 to Harui et al wherein a rotating ultrasound transducer is mounted in the scanhead of an endoscope. This patent discloses a transducer platform on which a multi-element transducer is assembled and fastened to a pulley. A pair of cables, extending from the endoscope handle controls, is then attached to this pulley to provide rotational movement of the transducer. The transducer electric wire bundle is complemented by a flexible printed circuit board (PCB). Although the transducer cable drive system as described in the Harui patent is relatively simple to build, the system must be maintained in constant cable tension in order to provide accurate rotation of transducer. Further, in a bendable endoscope, such cable drive device is fragile and inaccurate. Otherwise, no encoder is provided on the transducer, and the acoustic coupling method between transducer and the medium of interest is not addressed in the patent.
In U.S. Pat. No. 5,181,514 to Solomon et al and U.S. Pat. No. 5,176,142 to Mason, multiplane TE ultrasonic probes are disclosed which are an improvement over the probes of the Harui patent. An interconnect wire bundle management arrangement is disclosed, with a flexible PCB being located in two separated volumes provided in the endoscope transducer tip so that during rotation of transducer, the flexible PCB is transferred from one volume to the other via a take-up mechanism. A drive motor and encoder are mounted in the handle of endoscope. Rotational movement is transmitted to the scanhead via a flexible drive shaft. A gear assembly is provided in the vicinity of the transducer which mates with the drive shaft for the transducer. The Solomon et al patent also mentions the possibility of assembling both the motor and encoder in the transducer tip although no detailed description is provided in the patent and the mechanism described is explicitly designed for the endoscope handle. Further, the take-up mechanism for the flexible transducer PCB described therein results in a significant increase in the occupied volume in the tip housing that precludes or inhibits any possibility of motor integration in this part of the device. The position encoder, which includes a potentiometer as described in the patent, is of a construction incompatible with use thereof in a small TE probe tip volume. In addition, although a gearbox is provided to reduce the torque resistance transferred to the drive shaft, a variation in the friction force encountered by the transducer will produce torsional bending of the shaft, thereby leading to oscillations in the rotational speed of the transducer as well as significant motor current consumption, with the attendant risk of electrical noise appearing in the images produced.
In PCT application WO91/19458 to Ingebrigsten, a TE ultrasonic probe is disclosed which includes an endoscope distal housing comprising a rotatable shaft extending longitudinally along the housing, a motor for rotating the shaft and a multi-element ultrasonic transducer array connected to the shaft. The array is electrically connected by a first flexible PCB that extends, in S-like undulating manner, to a second flexible PCB which extends outwardly of the tip. The direct coupling between the transducer and motor eliminates the use of a flexible drive shaft; however, the output of the flexible transducer PCB exhibits a non-linear resistance force during operation resulting in an unreliable interconnection. Further, the technique disclosed of using a flexible PCB output prevents rotation of the transducer through a full turn. Finally, no rotation along the propagation axis of transducer is disclosed, thus dramatically limiting use of the device in TE modalities.
U.S. Pat. No. 5,445,154 to Larson et al discloses a multiplane ultrasonic probe wherein a cupped member is provided that receives a multi-element transducer array. The cupped member is mounted in a base unit via a low friction dynamic seal joint. The cupped member is attached to a drive gear which is engaged with a drive train. Rotation of the cupped member is produced by a ratchet gear driven by a pair of solenoids disposed in the handle of endoscope. A pulling or pushing action is transmitted to pawls by control cables. This method of driving the transducer in rotation is similar to those disclosed in the Harui patent and suffers from the same drawbacks described above. Further. The ratchet gear arrangement results in step by step rotation of the transducer and thus inhibits any 3D, on the fly acquisition. With regard to the dynamic seal mount, this requires the use of a large diameter seal and results in increased friction.
U.S. Pat. No. 5,456,256 to Schneider et al discloses an ultrasonic imaging system and apparatus wherein a sealing method is provided for separating the chamber for the liquid coupling from the outside. The transducer is typically a single element device and is mounted at the extremity of an oscillating arm. An oscillating platform is driven by a first motor which is directly coupled to the rotatable axis of the transducer arm. A second linear motorized drive is provided in the scanning plane in a manner such as to move the scanning line along the scanning surface. The transducer and arm are entirely immersed in the coupling liquid and the coupling bath is maintained in place by the use of a first oscillating sealed reservoir that surrounds the transducer area. Because the motor has to be isolated from the liquid, a second flexible seal is provided on the rotational axis of device. The second seal is designed such that a certain amplitude of alternate rotation of the axis is tolerated. This method is designed for scanning applications wherein dimensional constraint is not a concern. Moreover, moving such an arm in liquid will produce high movement resistance which is conducive to bubble formation in the liquid, resultant non-linear movement of transducer, or a significant variation in motor current when operating at high oscillating rates.
When considering the complexity of prior art TE ultrasound probes, and particularly those featuring a multiplane capability, it is evident that the prior art constructions of distal ultrasound transducer tips for endoscopic use suffer important disadvantages and, in general, are such as to make the instruments so specialized and intricate as to inhibit improvement on the quality and reliability thereof. Moreover, the annual maintenance of such prior art probe devices can cost up to one third of the purchase price, and despite the high quality of fabrication associated with such devices, the severe conditions to which the devices are subjected in medical applications has required that TE probes be mechanically inspected at least yearly in order to comply with current safety and performance specifications. There is, therefore, an obvious need for a TE ultrasonic diagnostic probe which has significantly improved reliability while, at the same time, decreasing the current purchase price and maintenance cost of such probes.